It was narrated that Anas said:
"The Messenger of Allah (ﷺ) commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once." (JA)
English reference (JA): Vol. 1, Book 7, Hadith 628 (JA)
Arabic reference (JA): Book 7, Hadith 634 (JA)